THE  UNIVERSITY 

OF  ILLINOIS 

LIBRARY 

630.7 
II6b 


co 


A6RI6ULIURAL 
U18ABY 


UNIVERSITY  OF  ILLINOIS    ' 

Agricultural  Experiment  Station 


BULLETIN  NO.  237 


EFFECT  OF  TEMPERATURE  OF 
PASTEURIZATION   ON   THE  CREAMING 
ABILITY  OF  MILK 


BY  H.  A.  HARDING 


URBANA,  ILLINOIS,  DECEMBER,  1921 


CONTENTS  OF  BULLETIN  237 

PAGE 
INTRODUCTION 395 

ESTABLISHMENT  OF  PASTEURIZING  TEMPERATURES 396 

DISTINCTION  BETWEEN  CREAM  LAYER  AND  CREAM  LINE 397 

THE  PRACTICAL  IMPORTANCE  OP  THE  CREAM  LAYER , 397 

MEASUREMENTS  OB'  CREAMING  ABILITY 398 

CORRECTIVE   EFFECT  OF  MOMENTARY  HEATING  ON  CREAMING 

ABILITY  OF  COLD   MILK 399 

THE  DESTRUCTIVE  EFFECT  OF  HIGHER  AND  MORE  PROLONGED 

HEATING 401 

LIMITATIONS  OF  TEMPERATURE  CONTROL 406 

SUMMARY 407 

RELATION  OF   THESE  FINDINGS  TO  THE  MILK  PROBLEM 407 

REFERENCES  .  .   408 


EFFECT  OF  TEMPERATURE  OF 

PASTEURIZATION  ON  THE  CREAMING 

ABILITY  OF  MILK 

BY  H.  A.  HARDING,  CHIEF  IN  DAIRY  BACTERIOLOGY 

INTRODUCTION 

Until  1900  commercial  pasteurization  was  a  compromise  between 
the  heating  which  the  scientist  considered  necessary  in  order  to  free 
milk  from  the  danger  of  carrying  disease  germs,  and  the  heating  which 
would  leave  the  milk  in  a  condition  acceptable  to  the  consumer. 
Under  these  circumstances,  milk  was  pasteurized  in  but  limited  quan- 
tities by  a  few  dealers,  and  the  use  of  the  pasteurized  product  was 
practically  restricted  to  infant  feeding. 

Since  the  recommendation  by  Russell  and  Hastings  in  19001  that 
pasteurization  at  140°  F.  for  thirty  minutes  be  accepted  as  satisfactory 
from  the  sanitary  standpoint,  commercial  pasteurization  has  spread 
over  the  country  with  ever  increasing  rapidity.  In  about  1913  pasteur- 
ization began  to  be  generally  recognized  as  the  most  important  safe- 
guard of  the  healthfulness  of  the  milk  supplies  of  the  country. 

The  principle  of  pasteurization  has  been  considered  settled  for 
more  than  two  decades.  However,  during  this  period  the  practice  of 
pasteurization  has  undergone  many  changes.  There  has  been  little 
question  but  that  the  bulk  of  pasteurized  milk  has  been  properly 
treated  in  the  various  pasteurizing  machines.  On  the  other  hand, 
criticism  regarding  minor  points  has  been  rather  constantly  raised. 
The  three  points  in  connection  with  which  pasteurizing  machines  are 
most  open  to  criticism  are:  (1)  the  lack  of  exactness  in  heating 
the  first  and  last  milk  during  the  act  of  pasteurizing;  (2)  the  occa- 
sional escape  of  milk  thru  leaky  valves;  and  (3)  the  results  from 
foam.  This  feeling  of  uncertainty  regarding  the  practical  details 
of  commercial  pasteurization  has  finally  led  to  a  definite  demand  for 
pasteurization  at  145°  F.  for  thirty  minutes  as  a  minimum  tempera- 
ture and  time.2 

This  movement  for  a  higher  temperature  in  pasteurization  has 
arisen  in  a  perfectly  natural  way  as  a  method  of  increasing  the  margin 
of  safety  surrounding  a  process  which  is  extremely  important  from 
the  standpoint  of  public  health.  In  view  of  the  fact  that  the  earlier 
attempts  to  pasteurize  milk  at  high  temperatures  produced  a  com- 
mercially unacceptable  product,  and  thereby  sharply  curtailed  the 
use  of  pasteurized  milk,  it  is  desirable  that  careful  study  be  given 

395 


396  BULLETIN  No.  237  [December, 

the  effect  upon  the  milk  of  the  suggested  change  in  pasteurizing  tem- 
perature to  a  minimum  of  145°  F.  The  recent  development  of  a 
method  by  which  the  creaming  ability  of  milk  can  be  easily  and 
accurately  measured  opens  the  way  for  a  study  of  this  problem.3 

Many  of  the  typical  pasteurizing  machines  have  a  capacity  of  10,000 
to  20,000  pounds  of  milk  per  hour.  The  high  cost  of  such  machines 
and  the  necessity  of  a  force  of  experienced  attendants  to  operate  and 
care  for  the  machinery,  together  with  the  need  of  a  large  supply  of  milk 
in  testing  the  machines  under  working  conditions,  made  it  practically 
imperative  that  such  a  study  be  conducted  in  commercial  milk  plants. 
Owing  to  the  generous  cooperation  of  the  Gridley  Dairy  Company  of 
Milwaukee,  the  Sheffield  Farms  Company,  Inc.,  of  New  York  City,  the 
Detroit  Creamery  Company  of  Detroit,  the  Pevely  Dairy  Company 
of  St.  Louis,  and  the  Bowman  Dairy  Company  of  Chicago,  it  was 
possible  in  this  study  to  cover  practically  all  the  prominent  types  of 
pasteurizing  machinery.  The  author  desires  to  express  appreciation 
of  the  generous  assistance  of  these  firms. 

ESTABLISHMENT  OF  PASTEURIZING  TEMPERATURES 

In  1895  when  Russell  proposed  pasteurization  of  milk  for  twenty 
minutes  at  155°  F.,4  there  was  some  uncertainty  regarding  the  time 
and  temperature  necessary  to  destroy  the  germs  of  tuberculosis.  The 
explanation  for  these  variations  in  scientific  results  was  found  by 
Theobald  Smith  in  1899.5 

The  problem  of  the  destruction  of  disease  germs  in  milk  by  pas- 
teurization was  again  studied  by  Russell  and  Hastings  in  1900,6  and 
by  Rosenau  in  1908.7  In  each  of  these  three  separate  investigations 
the  results  were  practically  identical. 

The  studies  of  Rosenau  showed  that  the  germ  of  tuberculosis  is 
much  more  resistant  to  heat  than  any  of  the  other  disease  germs  which 
occasionally  find  their  way  into  milk.  In  all  the  above  mentioned 
studies,  the  number  of  germs  of  tuberculosis  added  to  the  tested 
samples  of  milk  was  vastly  greater  than  would  be  present  as  the 
result  of  natural  infection.  Even  under  these  circumstances  there 
were  times  when  living  germs  of  tuberculosis  could  not  be  found  after 
the  milk  had  been  heated  to  140°  F.  for  one  minute.  Only  in  a  few 
cases  were  any  of  the  germs  alive  at  the  end  of  ten  minutes  of  heat- 
ing, and  in  no  case  were  any  found  after  heating  for  fifteen  minutes 
at  this  temperature. 

While  the  conclusions  from  these  three  investigations  were  iden- 
tical, there  are  interesting  differences  in  the  suggestions  of  the  investi- 
gators regarding  commercial  pasteurization.  Smith  made  no  recom- 
mendations on  this  subject.  Russell  and  Hastings  suggested  a  minimum 
of  140°  F.  for  twenty  minutes.  Rosenau,  with  identical  scientific 
results  before  him,  recommended  145°  F.  for  thirty  minutes. 


1921]     TEMPERATURE  OF  PASTEURIZATION  AND  CREAMING  ABILITY  OF  MILK         397 

The  feeling  is  common  among  those  who  have  carefully  considered 
commercial  pasteurization,  that  in  view  of  the  importance  of  the 
pasteurizing  process  from  the  standpoint  of  public  health  and  the 
imperfections  of  the  operation  and  supervision  of  pasteurizing 
machinery,  the  margin  of  safety  in  pasteurizing  time  and  temperature 
should  be  as  wide  as  the  production  of  a  satisfactory  commercial 
article  will  permit. 

DISTINCTION  BETWEEN  CREAM  LAYER  AND  CREAM  LINE 

At  present  the  phrase  "cream  line"  is  used  by  milkmen  in  refer- 
ring to  two  distinctly  different  things,  one  of  which  is  properly 
called  the  cream  line,  and  the  other  is  more  accurately  referred  to  as 
the  cream  layer. 

The  fat  globules  are  quite  evenly  distributed  in  freshly  drawn 
milk,  but  they  tend  to  rise  because  of  their  specific  gravity.  On 
standing  there  is  soon  formed  an  upper  layer  commonly  called  cream. 
The  relative  volume  of  the  cream  and  the  fat-poor  milk  below 
depends  largely  upon  the  fat  content  of  the  milk.  However,  there  are 
a  number  of  factors  which  at  times  markedly  change  the  normal  rela- 
tions. Important  among  these  factors  is  the  temperature  at  which 
milk  is  pasteurized. 

The  facility  with  which  milk  develops  this  cream  layer  is  referred 
to  as  creaming  ability.  In  bottled  milk  this  creaming  ability  is  com- 
monly expressed  in  terms  of  the  depth  of  the  layer  of  cream  in  the 
bottle  or  in  the  percentage  of  the  volume  of  the  bottle  occupied  by 
the  cream. 

As  the  cream  layer  forms  there  is  developed  a  more  or  less  dis- 
tinct line  of  demarkation  between  the  layer  of  cream  and  the  fat -poor 
milk  below.  This  line  of  demarkation  is  quite  properly  called  the 
cream  line.  Unless  a  cream  line  is  present  it  is  impossible  to  measure 
the  depth  of  the  cream  layer.  Because  the  -cream  line  and  the  depth 
of  the  cream  layer  are  so  closely  associated,  they  are  often  confused. 
It  is  important  that  they  be  considered  as  two  clearly  distinct  phe- 
nomena, because  the  influence  of  temperature  upon  the  two  is  quite 
different.  The  present  study  concerns  itself  with  the  effect  of  the 
temperature  of  pasteurization  upon  the  cream  layer. 

THE  PRACTICAL  IMPORTANCE  OF  THE  CREAM  LAYER 

The  cream  layer  is  important  for  two  reasons ;  first,  the  housewife 
uses  the  volume  of  cream  in  the  bottle  as  an  index  of  the  richness  of 
the  milk;  and  second,  she  has  need  of  cream  in  her  culinary  opera- 
tion and  she  is  accustomed  to  obtain  this  cream  from  the  top  of  the 
bottle. 

Until  the  studies  of  Theobald  Smith  in  1899  and  of  Russell  and 
Hastings  in  1900  made  it  evident  that  milk  would  be  safe  when 


398  BULLETIN  No.  237  [December, 

pasteurized  at  140°  F.,  commercial  pasteurization  made  practically 
no  progress  because  the  temperatures  previously  recommended  were 
such  as  to  practically  destroy  the  cream  layer. 

In  New  York  City  in  1914  the  health  authorities  required  pasteur- 
ization at  145°  F.  for  thirty  minutes,  but  the  difficulties  encountered 
by  the  milkmen  because  of  the  destructive  action  of  this  treatment 
upon  the  cream  layer  were  such  that  the  requirement  was  soon  modified 
to  that  of  142°  to  145°  F.  for  thirty  minutes.  At  present  there  are 
only  a  few  cities  which  require  the  pasteurization  of  their  ordinary 
milk  supplies.  In  the  remaining  cities  pasteurized  milk  is  sold  in 
competition  with  raw  milk.  Because  of  the  importance  ascribed  by 
the  consumer  to  the  cream  layer,  the  pasteurized  milk  which  fails  to 
carry  essentially  as  much  cream  as  the  raw  milk  does  not  meet  with 
popular  favor. 

It  is  agreed  by  practically  all  students  of  the  question  that  proper 
pasteurization  is  the  most  important  step  in  the  production  of  a  safe 
milk  supply.  An  efficient  pasteurization  is  exceedingly  important,  but 
it  is  also  important  that  the  relation  of  pasteurization  to  the  cream 
layer  be  understood,  so  that  in  the  zeal  to  provide  as  wide  a  margin 
of  safety  as  possible  the  increase  in  pasteurization  temperature  will 
not  be  carried  so  far  beyond  the  actual  requirements  of  the  case  as 
to  reduce  the  use  of  safe,  pasteurized  milk. 

MEASUREMENTS  OF  CREAMING  ABILITY 

The  various  methods  of  measuring  the  creaming  ability  of  milk 
are  discussed  at  length  in  Circular  249  of  this  Station. 

Three  of  these  methods  are  adapted  to  the  measurement  of  cream 
which  has  risen  in  the  milk  bottle.  The  simplest  of  these  is  to  stand 
bottles  beside  each  other  and  compare  the  depth  of  the  cream  layers. 
Another  method  is  to  measure  the  distance  from  the  top  of  the  bottle 
to  the  line  dividing  the  cream  from  the  milk.  More  accurate  compari- 
sons may  be  made  by  determining  the  volume  occupied  by  the  layer 
of  cream.  Where  the  total  volume  of  the  bottle  is  known,  the  content 
of  cream  may  be  expressed  in  percentage  by  volume.  The  first  two 
methods  are  quite  inaccurate  and  the  third  is  rather  laborious. 

In  the  present  study  the  measurements  of  creaming  ability  were 
made  by  filling  round-bottomed  test  tubes,  one-inch  in  diameter,  to 
a  depth  of  216  millimeters  (81/2  inches),  with  the  milk  to  be  tested. 
These  tubes  of  milk  were  immediately  cooled  in  ice  water,  and  when 
cool  were  held  at  40°  F.  for  approximately  twenty-four  hours.  The 
depth  of  the  resulting  cream  layer  was  measured  in  millimeters,  and 
each  millimeter  of  cream  represented  0.47  percent  by  volume. 

The  volume  of  cream  as  determined  in  this  way  agrees  closely  with 
the  volume  of  cream  developed  in  bottles  under  similar  temperature 
conditions. 


TEMPERATURE  OF  PASTEURIZATION  AND  CREAMING  ABILITY  OF  MILK 


399 


CORRECTIVE   EFFECT   OF  MOMENTARY  HEATING  UPON 
THE  CREAMING  ABILITY  OF  COLD  MILK 

The  temperature  of  milk  when  drawn  from  the  cow  is  about  100°  F. 
The  best  dairy  practice  aims  to  bring  the  milk  promptly  to  a  tem- 
perature of  50°  F.  or  below.  During  the  colder  months,  the  tempera- 
ture of  the  milk  often  falls  nearly  or  quite  to  the  freezing  point  before 
it  reaches  the  milk  plant. 

TABLE  1. — EFFECT  OF  MOMENTARY  HEATING  UPON  THE  CREAMING  ABILITY  OF  MILK 


Date 

Raw  milk 

Heated  milk 

Temperature 

Cream 

Temperature 
heated 

Cream 

Change 

op 

% 

o  p 

% 

% 

Apr.     12 

39 

11.32 

85 

13.21 

+  1.89 

May     11 

40 

9.43 

141 

12.26 

+  2.83 

Apr.     21 

41 

8.07 

132 

12.26 

+  4.19 

Apr.       4 

42 

9.90 

140 

11.79 

+  1.89 

June    16 

45 

11.32 

142 

13.21 

+  1.89 

June   16 

47 

11.32 

142 

13.68 

+  2.36 

June    1  7 

48 

11.87 

142 

13.21 

+  1.34 

Oct.     25 

48        t 

11.79 

143 

13.21 

+  1.42 

Feb.     19 

49 

7.55 

90 

10.85 

+  3.30 

Oct.     26 

49 

13.68 

143 

14.62 

+    .94 

Feb.       4 

50 

11.79 

96 

14.15 

+2.36 

June   18 

50 

11.32 

142 

12.74 

+  1.42 

Oct.     24 

51 

12.74 

143 

12.97 

+    .23 

Oct.     25 

51 

12.26 

143 

11.02 

—  1.24 

Oct.     26 

51 

14.38 

143 

14.15 

—   .23 

Oct.     26 

51 

14.62 

143 

14.15 

—    .47 

Oct.     26 

51 

13.68 

143 

12.97 

—    .71 

Oct.     26 

51.5 

14.15 

143 

14.15 

.00, 

Oct.     25 

52 

12.97 

143 

13.21 

+    .24 

Oct.     26 

52 

12.97 

143 

13.91 

+    .94 

Oct.     26 

53 

13.91 

143 

13.21 

—    .70 

Oct.     25 

53.5 

12.50 

143 

12.50 

.00 

Oct.     24 

54 

12.74 

143 

12.74 

.00 

Oct.     24 

54 

12.26 

143 

12.74 

+    .48 

Oct.     25 

54 

13.44 

143 

12.97 

—    .47 

Oct.     26 

54 

12.74 

143 

13.91 

+  1.17 

Oct.     26 

54 

13.91 

143 

13.91 

.00 

Oct.     26 

54 

14.62 

143 

14.15 

—    .47 

Oct.     24 

55 

14.85 

143 

14.38 

—    .47 

Oct.     25 

55 

12.50 

143 

11.02 

—  1.48 

Apr.       2 

56 

14.62 

85 

14.15 

—    .47 

Oct.     24 

56 

13.91 

143 

13.68 

—    .23 

Oct.     25 

56 

12.97 

143 

13.21 

+    .24 

Oct.     26 

56 

14.15 

143 

12.74 

—  1.41 

Apr.    22 

57 

14.15 

129 

14.15 

.00 

Oct.     24 

57 

15.09 

143 

14.85 

—    .24 

Oct.     26 

57 

14.15 

143 

13.44 

—    .71 

Apr.     23 

58 

11.79 

131 

11.79 

.00 

Oct.     24 

58 

14.62 

143 

13.68 

—    .94 

Oct.     25 

58 

13.91 

143 

12.74 

—  1.17 

Oct.     25 

58 

13.68 

143 

12.26 

—  1.42 

Oct.     25 

58.5 

13.21 

143 

12.26 

—    .95 

Apr.     21 

59 

12.26 

133 

12.26 

.00 

June    24 

66 

15.09 

141 

13.68 

—  1.41 

June    25 

68 

15.57 

143 

13.68 

—  1.89 

June    26 

69 

15.57 

142 

13.68 

—  1.89 

400  BULLETIN  No.  237  [December, 

Measurements  of  the  creaming  ability  of  cold  milk  as  delivered 
at  milk  plants,  together  with  measurements  of  the  creaming  ability 
of  the  same  milk  after  momentary  heating  to  various  temperatures, 
are  given  in  Table  1. 

These  data  were  derived  from  observations  at  six  widely  separated 
milk  plants.  While  the  fat  content  of  the  milk  received  at  these  plants 
averaged  between  3.4  and  3.6  percent,  that  of  individual  samples 
varied  more  widely.  The  treatment  of  the  milk  previous  to  sampling 
is  not  known  in  detail,  but  the  conditions  varied  from  that  of  milk 
which  was  being  delivered  from  nearby  farms  to  that  of  milk  which 
had  been  delivered  at  country  milk  plants,  cooled  to  about  40°  F., 
and  shipped  by  rail  to  the  city  plant.  Accordingly,  there  was  a  wide 
variation  in  the  temperatures  to  which  the  milk  had  been  cooled  and  in 
the  agitation  to  which  it  had  been  exposed  while  cold. 

The  results  presented  in  Table  1  are  arranged  according  to  the 
temperature  of  the  milk  at  the  receiving  vat.  There  Is  no  evident 
relation  between  the  creaming  power  of  the  cold  milk  and  the  tem- 
perature at  which  it  was  received,  except  that  the  three  samples 
received  above  65°  F.  all  show  a  high  creaming  power.  These  samples 
represent  milk  coming  directly  from  nearby  farms.  The  four  samples 
of  raw  milk  which  developed  less  than  10  percent  of  cream  by  volume 
came  from  four  different  plants.  In  each  case  they  represent  milk 
which  had  either  been  held  cold  for  considerable  time  at  the  plant, 
with  prolonged  agitation,  or  had  been  subjected  while  cold  to  cor- 
responding agitation  during  transportation. 

After  leaving  the  receiving  vat  the  milk  was  heated,  in  some  cases 
by  a  forewarmer  and  in  other  cases  by  a  pasteurizer.  Samples  were 
collected  and  the  creaming  power  determined  after  this  momentary 
heating. 

In  the  case  of  the  milk  received  at  or  below  50°  F.,  a  momentary 
heating  ranging  from  85°  to  142°  F.  resulted  in  a  measurable  increase 
in  the  creaming  power.  On  the  other  hand,  when  the  temperature  of 
the  milk  on  its  receipt  was  between  50°  and  60°  F.,  a  similar  heating 
usually  produced  little  change ;  and  when  the  temperature  on  receipt 
was  above  65°  F.,  such  heating  resulted  in  a  distinct  decrease  in  the 
creaming  ability. 

In  observing  the  effect  of  any  given  treatment  upon  the  creaming 
power,  such  treatment  is  frequently  judged  by  the  relative  amounts 
of  cream  obtained  upon  the  raw  and  upon  the  treated  milk.  The 
above  data  suggest  that  the  amount  of  cream  which  will  develop  upon 
raw  milk  depends  quite  largely  upon  the  agitation  to  which  it  has 
been  subjected  while  cold. 

The  results  given  in  Table  1  show  that  milk  with  a  comparable  fat 
content,  when  moderately  heated,  is  given  a  fairly  comparable  cream- 
ing power  regardless  of  the  agitation  to  which  it  has  been  exposed 


1921]     TEMPERATURE  OF  PASTEURIZATION  AND  CREAMING  ABILITY  OF  MILK         401 

while  cold.  A  more  detailed  study  of  this  phase  of  the  question  is 
necessary  before  it  will  be  certain  that  the  creaming  power  thus 
attained  is  identical  with  the  original  creaming  power  of  the  fresh 
milk.  In  fact,  the  results  from  the  last  three  samples  suggest  that 
the  creaming  power  of  fresh  warm  milk  is  slightly  reduced  by  a 
momentary  heating  to  141°-143°  F. 

This  corrective  effect  of  momentary  heating  upon  the  creaming 
ability  of  milk  is  quite  important  in  connection  with  the  study  of 
the  effect  of  the  pasteurizing  process.  Thru  this  corrective  action,  the 
effect  of  earlier  agitation  while  the  milk  is  cold  is  largely  removed 
and  the  milk  is  given  a  fairly  uniform  creaming  power.  Under  such 
circumstances  the  variations  in  the  creaming  power  of  pasteurized 
milk  having  the  same  fat  content  are  practically  all  due  to  the  tem- 
peratures used  in  pasteurization. 


The  measurements  in  this  study  were  made  in  the  plants  earlier 
mentioned,  partly  because  they  were  representative  plants  and  partly 
because  the  observations  could  thus  be  made  to  include  practically  all 
the  systems  of  pasteurization  now  in  commercial  use.  There  was  also 
in  mind  the  common  belief  that  each  different  type  of  machine  has  its 
own  peculiar  effect  upon  the  volume  of  cream  which  will  develop. 

In  this  study  samples  were  collected,  at  convenient  intervals,  all 
the  way  from  the  receiving  vat  up  to  and  including  the  bottles. 
Samples  were  taken  in  many  instances  after  the  milk  had  been  held 
at  the  given  temperature  for  periods  ranging  from  five  minutes  to  an 
hour,  but  the  study  was  centered  upon  the  effect  after  a  holding 
period  of  thirty  minutes. 

The  samples  obtained  in  commercial  plants  were  taken  under 
regular  working  conditions,  except  that  the  temperature  of  pasteuri- 
zation was  deliberately  varied  from  the  lowest  temperature  consistent 
with  safety  and  with  the  local  ordinances,  up  to  145°-149°  F.  All 
temperatures  shown  on  the  charts  are  those  at  the  close  of  the  thirty- 
minute  holding  period.  In  some  plants  the  lower  limit  was  140°  F., 
while  in  others  it  was  142°  F.  In  the  plants  of  the  Sheffield  Farms 
Company  Inc.  no  samples  were  obtained  after  pasteurizing  at  a  tem- 
perature of  less  than  145°  F. 

Ideal  conditions  for  testing  the  effect  of  the  pasteurizing  tempera- 
ture upon  the  creaming  power  of  milk  would  include  a  sufficient  supply 
of  raw  milk  of  uniform  composition  so  that  it  could  be  run  thru  a 
single  pasteurizer  with  all  conditions  kept  constant  except  the  tem- 
perature of  pasteurization.  In  commercial  plants  the  most  common 
difficulty  is  the  variation  in  the  fat  content  of  the  raw  milk  supply. 


402 


BULLETIN  No.  237 


[December, 


o    13.68 

s 

>     12.74 

>> 

,0 

S     11.79 

P 

5 
-g     10.85 

i 

c 

£      9.90 

.       1' 

Fig.   1.— 

t 

utes  at 

now  ever,    very    suus- 
f  actory  conditions  for 
the  study  were  found 
in  a  plant  in  which 
the  milk  was  received 
from  two  country  sta- 
tions,   where    it   had 
been  cooled  to  about 
40     F.   before  being 
shipped   to    the    city 
plant.      In    the    city 
plant  it  was  held  cold 
in  large  tanks  until 
needed.     The  results 
1S      of  forty  tests  of  the 
result  of  pasteuriza- 
tion for  thirty  min- 
.  to  146°  F.  are  given  in 

L~^, 

1^. 

£      .. 

N.. 

^1 

Si       i 

\ 

V 

X 

N 

10°                  142°                  144°           146°  F 

Forty  Tests  of  the  Effect  of  Pasteurizii 
Temperatures  on  the  Cream  Layer 

temperatures  ranging  from  140°  F 

Fig.  1. 

In  Fig.  1  each  dot  shows  the  result  from  a  single  test,  and  by  its 
position  indicates  the  temperature  of  pasteurization  and  the  percentage 
of  cream  by  volume  which  developed.  For  example,  the  dot  at  the 
extreme  left  indicates  that  a  sample  of  milk  pasteurized  for  thirty 
minutes  at  140°  F.  developed  12.74  percent  of  cream  by  volume. 

The  line  running  across  the  chart  connects  the  averages  of  the 
results  at  each  temperature  and  shows  their  relation.  This  line  in 
Fig.  1  indicates  that  there  is  a  distinct  difference  between  the  effect 
of  pasteurization  at  142°  F.  and  at  144°  F.  upon  the  creaming  ability 
of  the  milk.  As  the  temperature  of  pasteurization  rises  above  144°  F., 
the  volume  of  cream  decreases  rapidly. 

A  similar  group  of  results  was  obtained  in  a  milk  plant  in  another 
city,  where  the  milk  on  receipt  was  held  in  large  tanks  before  pas- 
teurization. In  this  plant  the  observations  extended  over  tempera- 
tures ranging  from  140°  to  151°  F.  The  results  from  forty-five  such 
measurements  are  shown  in  Fig.  2. 

It  will  be  seen  from  Fig  2  that  between  142°  and  144°  F.  there 
is  a  distinct  change  in  the  direction  of  the  line  connecting  the  averages 
at  each  temperature,  and  this  sharp  decrease  in  the  volume  of  the 
cream  continues  rather  uniformly  up  to  a  pasteurization  for  thirty 
minutes  at  151°  F.  At  this  latter  temperature,  the  resulting  cream 
amounts  to  less  than  4  percent. 

It  should  be  noted  in  this  connection  that  altho  pasteurization  at 
151°  F.  had  reduced  the  volume  of  cream  more  than  80  percent,  a 
distinct  cream  line  was  still  formed.  Such  observations  make  it  plain 
that  the  temperature  of  pasteurization  exerts  a  powerful  influence 


1981]     TEMPERATURE  OF  PASTEURIZATION  AND  CREAMING  ABILITY  OF  MILK         403 


12.74 

11.79 

g     10.85 


9.90 


8.96 
8.02 
7.08 
6.13 
5.19 
4.18 


\ 


\ 


\ 


X 


\ 


140C 


142 c 


144 c 


146 c 


148C 


150°  F. 


upon  the  vol- 
ume of  cream 
even  before  it 
reaches  the 
point  where 
i  t  destroys 
the  cream 
line  entirely. 
In  the  two 
plants  from 
which  results 
have  been 
presented  the 
milk  was  of 
r  el  a  t  ively 
uniform  fat 
content  b  e- 
fore  pasteuri- 
zation. In  a 
plant  in  a 
third  city  the 

conditions  were  equally  satisfactory,  except  that  owing  to  the  nature 

of  the  supply  there  was  more  variation  in  the  fat  content  during  the 

day's  run. 

The    results    from 

fifty-five  tests  of  pas- 
teurization at  tem- 
peratures ranging 

from  140°  to  146°  F., 

in  the  third  plant,  are 

shown  in  Fig.  3. 
It  will  be  seen  from 

Fig.  3  that  the  results 

from  this  plant  were 

very  similar  to  those 

obtained     from     the 

two    plants    already 

mentioned.    This  fact 

is  particularly  strik- 


Fig.  2. — Forty-five  Tests  of  the  Effect  of  Pasteurizing  Tem- 
peratures on  the  Cream  Layer 


12.74 


11.79 


10.85 


9.90 


8.96 


Fig.  3. — Fifty-five  Tests  of  the  Effect  of  Pasteurizing 
Temperatures  on  the  Cream  Layer 


140°  142°  144°  146°  F. 

ing    because    of    the 

fact  that  each  plant 
operated  a  distinctly 
different  kind  of  pasteurizer.  It  will  be  noted  that  there  is  again  a 
distinct  decrease  in  the  volume  of  the  cream  layer  between  pasteuriza- 
tion at  142°  F.  and  at  144°  F.,  and  this  decrease  continues  as  the 
temperature  of  pasteurization  is  increased. 


404 


BULLETIN  No.  237 


[December, 


11.79 


9.90 


140C 


142 


Fig.  4. — Ninety-eight  Tests  of  the  Effect  of  Pasteur- 
izing Temperatures  on  the  Cream  Layer 


All  the  foregoing 
results  were  obtained 
at  city  plants.  At 
country  plants,  close 
to  the  point  of  pro- 
duction, the  pasteur- 
izing machinery  is 
started  as  soon  as  a 
small  supply  of  milk 
is  at  hand,  and  at 
times  the  milk  in  the 
receiving  vat  may  be 
that  of  only  one  or 
two  patrons.  Under 
such  conditions  there 

is  a  maximum  variation  in  the  fat  content  and  a  corresponding  varia- 
tion in  the  results  of  the  measurements. 

The  results  of  ninety-eight  tests  at  such  a  country  plant  are  shown 
in  Fig.  4. 

Owing  to  a  number  of  disturbing  factors,  the  results  shown  in 
Fig.  4  are  somewhat  less  accordant  than  those  previously  shown.  It 
chanced  that  about  one-third  of  the  patrons  of  this  milk  plant  used 
milking  ma- 
chines,  and 
about  one-half 
of  the  tests  were 
made  at  the  time 
of  the  first  hot 
wave  of  the  sea- 
son, when  the 
milk  was  unduly 
acid.  The  re- 
maining obser- 
v  a  t  i  o  n  s  were 
made  about  one 
month  later, 
when  conditions 
were  normal. 
The  results  of 
this  second  visit, 
taken  by  them- 
selves, would 
show  a  normal 
temperature  re- 
lation. 


14.62 
13.68 
12.74 

c,  11-79 

8 
.3  10.85 

o 

^    9.90 

fi 

S    8.96 

03 

5    8.02 
«     7.08 

Ci 

&     6.13 
5.19 
4.24 

• 

• 

-•    . 

•i-j: 

*          If 

-•:    : 

.. 

r*  —  •• 

5  BJ 

"^ 

*.    - 

. 

•    :: 

C3 

SL  .. 

~ 

•  • 

^ 

• 

• 

*          • 

N. 

I    S^ 

7 

•' 

\ 

'\ 

\ 

> 

\ 

\ 

\ 

\ 

^ 

V 

\ 

v^ 

t 

\ 

\ 

\ 

140C 


142C 


144C 


146  = 


148° 


150°  F. 


Fig.  o. — Two  Hundred  Eight  Tests  of  the  Effect  of  Pasteur- 
izing Temperatures  on  the  Cream  Layer 


1921]     TEMPERATURE  OF  PASTEURIZATION  AND  CREAMING  ABILITY  OF  MILK         405 


14.62 


13.68 
12.74 

n7Q 
—  •  i  v 

i 

|  10.85 
^  9.90 
S  8.96 

02 
V 

O       8.02 

41 

I       7.08 


6.13 


5.19 

4.24 


A 


N 


x 


\ 


\ 


140 c 


142 c 


144C 


146 c 


148 c 


150°  F. 


Fig.  6. — Summary  of  401  Tests  of  the  Effect  of  Pasteurizing 
Temperatures  on  the  Cream  Layer 


These  re- 
sults empha- 
size the  fact 
that  while  the 
temperature 
of  pasteuriza- 
tion may  be, 
and  often  is, 
the  largest 
factor  in  de- 
termining the 
volume  of 
cream  in  the 
milk  bottle, 
this  cream 
volume  is  also 
influenced  by 
other  factors 
which  are 
still  for  the 
most  part  lit- 
tle under- 
stood. 


All  the  results  thus  far  presented  were  obtained  by  the  use  of  a 
single  pasteurizing  machine  in  each  plant.  Many  of  the  larger  plants 
are  equipped  with  two  or  more  machines.  In  Fig.  5  are  given  the 
results  from  208  measurements,  which  include  samples  from  five  dif- 
ferent pasteurizers  in  two  adjoining  plants  handling  a  common  milk 
supply.  Three  of  these  machines  were  of  the  same  make,  but  these 
three  machines  varied  as  widely  in  their  effect  upon  the  creaming 
power  of  the  milk  handled  in  them  as  did  the  three  machines  of  dif- 
ferent makes.  The  differences  in  the  effects  from  the  five  machines 
were  really  very  slight.  The  data  from  one  of  these  machines  have 
already  been  presented  in  Fig.  2.  The  results  shown  in  this  chart 
are  more  varied  than  those  previously  shown.  This  is  due  in  part 
to  the  larger  number  of  observations,  in  part  to  the  effect  of  the 
different  machines,  and  in  part  to  the  fact  that  these  tests  extended 
over  a  period  of  about  four  months. 

However,  the  general  effect  of  pasteurization  upon  the  cream  is 
practically  identical  with  that  found  in  the  other  plants  already  dis- 
cussed. The  volume  of  cream  begins  to  decrease  noticeably  at  tem- 
peratures between  142°  and  144°  F.,  and  this  decrease  grows  rapidly 
with  the  increase  in  the  temperature  of  pasteurization. 

The  results  presented  in  Figs.  1  to  5,  showing  the  effect  of  the 
temperature  of  pasteurization  upon  the  volume  of  cream  later  de- 


406  BULLETIN  No.  237  [December, 

veloped  on  the  milk,  were  taken  from  five  different  plants.  These 
plants  were  selected  from  a  larger  number  in  which  tests  have  been 
made,  because  in  each  of  these  plants  the  tests  were  sufficient  in  num- 
ber to  provide  a  basis  for  a  dependable  estimate  of  the  effect  of  the 
pasteurizing  temperatures  used,  and  because  in  each  of  them  the  aver- 
age fat  content  of  the  milk  handled  ranged  between  3.4  and  3.6  per- 
cent, altho  the  individual  samples  varied  more  widely.  The  data 
from  other  plants,  so  far  as  they  go,  were  entirely  in  harmony  with 
those  here  presented. 

Since  these  data,  while  accumulated  in  widely  scattered  cities,  are 
really  quite  comparable,  the  results  of  the  401  tests  are  grouped 
in  Fig.  6. 

While  the  results  shown  in  Fig.  6  vary  considerably,  the  fact  stands 
out  clearly  that  the  cream  rising  on  the  milk  pasteurized  at  142°  F. 
was  distinctly  more  abundant  than  that  rising  on  the  milk  pasteurized 
at  144°  F.  As  the  temperature  of  pasteurization  rises  above  144°  F., 
the  decrease  in  cream  becomes  rapidly  more  pronounced.  Taking  the 
volume  of  cream  obtained  at  142°  F.  as  the  basis  of  calculation,  the 
loss  in  volume  as  the  pasteurizing  temperature  is  increased  to  145°  F. 
amounts  to  slightly  more  than  10  percent ;  at  146°  F.,  it  has  increased 
to  16.6  percent ;  and  at  148°  F.,  it  has  increased  to  approximately  40 
percent  by  volume. 


LIMITATIONS  OF  TEMPERATURE  CONTROL 

Milk  pasteurization  is  frequently  spoken  of  as  tho  it  were  con- 
ducted at  a  fixed  temperature.  While  this  represents  an  ideal  toward 
which  the  industry  is  striving,  it  is  one  which  has  not  been  realized. 

The  details  of  temperature  control  vary  with  the  different  types 
of  pasteurizing  machinery.  The  vat  pasteurizers  depend  largely  upon 
direct  control  by  the  operator,  while  the  continuous  machines  are 
commonly  provided  with  automatic  devices.  However,  these  automatic 
regulators  require  continuous  oversight  and  adjustment,  and  at  best 
the  temperature  fluctuations  are  merely  kept  within  more  narrow 
limits. 

In  extreme  cases  the  limits  of  temperature  fluctuation  may  be  as 
wide  as  ten  degrees;  more  commonly  they  do  not  amount  to  more 
than  five  degrees,  and  in  well  regulated  plants  the  variation  during 
the  day's  run  may  be  held  down  to  about  three  degrees.  It  is  very 
rare  indeed  that  fluctuations  in  the  temperature  of  pasteurization  for 
the  entire  day  are  consistently  kept  within  any  narrower  limits. 

The  phrase  "pasteurized  at  142°-145°  F.,  for  thirty  minutes"  is 
accurately  descriptive  of  the  work  in  many  of  the  best  milk  plants. 
In  such  plants  the  bulk  of  the  milk  is  pasteurized  at  144°  F.,  with 
occasional  brief  deviations  in  either  direction.  In  the  plants  having 


TEMPERATURE  OF  PASTEURIZATION  AND  CREAMING  ABILITY  OF  MILK         407 

the  most  perfect  temperature  control  of  any  known  to  the  author, 
the  temperature  fluctuations  amount  to  about  two  degrees.  In  addi- 
tion to  a  constant  and  exacting  personal  supervision,  this  perfection 
of  temperature  control  has  been  preceded  by  an  initial  cash  outlay 
for  plant  and  equipment  of  well  above  a  half -million  dollars. 

Few  milk  companies  have  an  amount  of  business  and  a  financial 
backing  which  will  permit  such  heavy  investment.  Altho  it  is  to  be 
hoped  that  the  same  accuracy  of  temperature  control  may  be  attained 
thru  less  expensive  installation,  the  fact  seems  to  be  that  so  far  it 
has  not  been  thus  attained. 

SUMMARY 

The  layer  of  cream  on  the  bottle  of  milk  as  delivered  to  the  con- 
sumer is  important  because  the  consumer  uses  it  as  an  index  of  the 
richness  of  the  milk  and  as  a  source  of  cream. 

Since  1900  a  heating  of  the  milk  to  140°-145°  F.  for  thirty  minutes 
has  been  generally  recognized  as  proper  pasteurization.  Recently 
there  has  been  a  growing  demand  that  the  temperature  of  pasteuriza- 
tion be  increased  to  145°  F.  for  thirty  minutes  as  a  minimum  time 
and  temperature. 

In  the  operation  of  practically  all  large  commercial  pasteurizers, 
a  variation  of  at  least  three  degrees  Fahrenheit  is  practically  unavoid- 
able. Many  machines  vary  even  more  widely.  Accordingly,  the 
requirement  of  145°  F.  as  the  minimum  temperature  means  pasteuriza- 
tion at  145°  to  148°  F. 

The  data  here  presented  show  that  the  volume  of  creafti  on  milk 
begins  measurably  to  decrease  when  the  temperature  of  pasteurization 
rises  from  142°  F.  to  144°  F.  As  the  temperature  goes  higher,  the 
decrease  in  the  volume  of  cream  becomes  rapidly  more  pronounced; 
at  145°  F.  it  amounts  to  slightly  more  than  10  percent  by  volume; 
at  146°  F.  it  amounts  to  16.6  percent ;  and  at  148°  F.  to  approximately 
40  percent. 

RELATION  OF  THESE  FINDINGS  TO  THE  MILK  PROBLEM 

As  has  been  pointed  out,  without  the  presence  of  a  satisfactory 
layer  of  cream  in  the  top  of  the  milk  bottle  it  is  difficult  to  induce 
the  public  to  purchase  pasteurized  milk. 

The  results  here  presented  show  that  when  the  temperature  of 
pasteurization  is  raised  to  144°  F.,  the  volume  of  cream  begins 
measurably  to  decrease.  As  the  temperature  of  pasteurization  is  fur- 
ther increased,  the  damage  to  the  cream  layer  increases  rapidly. 

It  has  been  further  pointed  out  that  there  is  an  unavoidable  fluc- 
tuation in  the  temperature  of  pasteurization,  and  that  this  fluctuation 
rarely  amounts  to  less  than  three  degrees  and  often  ranges  more 


408  BULLETIN  No.  237 

widely.  Accordingly,  if  pasteurization  is  defined  as  heating  to  145°  F., 
as  a  minimum,  for  thirty  minutes,  such  pasteurization  involves  heating 
the  milk  to  145°-148°  F.,  and  in  many  plants  will  involve  even  higher 
temperatures. 

Taking  these  two  sets  of  facts  together  it  is  evident  that  the  re- 
quirement that  milk  be  pasteurized  at  a  minimum  temperature  of 
145°  F.  for  thirty  minutes  will  result  in  a  sharp  reduction  of  the 
cream  layer.  This  conclusion  is  not  only  to  be  drawn  from  the  fore- 
going studies,  but  it  is  also  in  entire  accord  with  practical  experience. 

The  pasteurizing  process  is  the  most  important  protection  which 
the  public  has  against  the  spread  of  disease  thru  milk.  It  is  therefore 
important  that  this  process  carry  the  broadest  practicable  margin  of 
safety.  At  the  same  time  it  is  equally  important  that  the  margin  of 
safety  should  not  be  carried  to  such  limits  as  to  produce  a  milk 
unacceptable  to  the  public,  and  thereby  reduce  .the  consumption  of 
safe,  pasteurized  milk. 

Repeated,  careful  studies  by  different  scientists  have  uniformly 
shown  that  the  most  resistant  disease  germs  which  may  enter  milk 
are  destroyed  by  a  fifteen -minute  heating  at  140°  F.  Under  such 
circumstances  pasteurization  at  142°-145°  F.  for  thirty  minutes  pro- 
vides a  very  considerable  margin  of  safety,  both  in  the  matter  of  tem- 
perature and  of  time  of  exposure. 

The  proposition  to  require  pasteurization  at  a  minimum  of  145°  F. 
for  thirty  minutes  involves  so  large  a  destruction  of  the  cream  layer, 
which  is  highly  esteemed  by  the  public,  that  such  requirement  would 
probably  result  in  a  decreasing  consumption  of  pasteurized  milk. 
Accordingly,  this  increase  in  the  margin  of  safety  w7ould  hardly  seem 
justified  in  the  absence  of  any  evidence  that  142°-145°  F.  is  insuffi- 
cient, and  in  the  absence  of  any  attempt  to  provide  increased  safety 
in  other  ways. 

REFERENCES 

1.  RUSSELL,  H.  L.,  AND  HASTINGS,  E.  p.    Thermal  death  point  of  tubercle  bacilli 
under  commercial   conditions.     Wis.   Agr.   Exp.   Sta.   Ann.  Eept.   17    (1900), 
147-170.     1900. 

2.  Report  of  Committee  on  Milk  Supply,  Sanitary  Engineering  Section,  American 
Public  Health  Association.     August,  1920. 

3.  HARDING,  H.  A.     The  measurement  of  the  volume  of  cream  on  milk.     111.  Agr. 
Exp.  Sta.  Circ.  249.     1921. 

4.  RUSSELL,  H.  L.     Pasteurization  of  milk  and  cream  for  direct  consumption. 
Wis.  Agr.  Exp.  Sta.  Bui.  44.     1895. 

5.  SMITH,  THEOBALD.     The  thermal  death  point  of  the  tubercle  bacillus  in  milk 
and  some  other  fluids.     Jour,  of  Exp.  Med.  4,  217-233.     1899. 

6.  See  1,  above. 

7.  ROSENAU,  M.  J.     Thermal  death  points  of  pathogenic  microorganisms  in  milk. 
Public  Health  and  Marine  Hospital  Service  of  the  U.  S.    Hvgicnic  Laboratory 
Bui.  42.     1908. 


UNIVERSITY  OF  ILLINOIS-URBAN* 


